parallax

(angle due to different lines of sight)

Parallax is the angle between the apparent location
of an object as seen from two different places.
It is used to measure their distance to stars.
If a very small angle can be measured accurately,
a distance to a nearby star can be determined.

For measuring distances to a star, the parallax used
is the angle between viewing the star from Earth at
two times, half a year apart, when the positions
of the Earth differ by 2 AU (the baseline). The angle cited as parallax
(parallax angle)
is typically half this, the angle from two positions
1 AU apart, e.g., from the Sun and the Earth.
A parsec is the distance of a star with this (1 AU) parallax
angle of 1 arcsecond. Typical capabilities of telescopes: